1. Field of the Invention
This invention relates to flexible shaft couplings having intermeshing gear teeth and more particularly to flexible gear type couplings having a shear spacer adapted to shear when excessive torque is applied to the coupling and a capturing device for maintaining the sleeve of the coupling substantially concentric around the hub when the shear spacer shears.
2. Description of the Prior Art
In any coupling and in particular a flexible gear type coupling the function thereof is to transmit torque between a pair of substantially coaxially aligned shafts while at the same time accommodating any axial, angular, and parallel offset misalignment that exists between the shafts. The amount of torque to be transmitted by the coupling is determined by the amount that can be accepted by the shafts and the driven apparatus without any damage being done. Since couplings are relatively expensive, they are designed to transmit that torque which is to be accepted by the shafts and the driven apparatus. Should a coupling designed to transmit a small amount of torque be used between the shafts that are designed to transmit a great amount of torque, the meshing gear teeth in the flexible coupling may become damaged when excess torque is applied to the coupling.
Further, should a coupling designed to accommodate a large torque be used with a device in which only small torques are to be used, the coupling is overdesigned for the job. In addition, if a larger torque than desired is momentarily transmitted, the larger coupling will transmit it to the device which may be damaged by the excessive torque.
Even when a gear type coupling has been properly designed to take the specified amount of torque to be transmitted, it often develops that in a given situation or at a given moment in time, a torque greater than the amount designed to be transmitted through the coupling may be applied to the driving shaft. When this occurs, the meshing gear teeth of the coupling may be damaged by the excessive torque requiring the coupling to be replaced. Since it is extremely difficult to design an apparatus in which it can be assured that no excess torque will be transmitted to the driven device, couplings have been developed that contain shear pins connecting the various elements in the coupling. When an excessive torque is applied to the coupling, the shear pin shears, disconnecting the driving shaft from driven shaft thus protecting both the intermeshing gear teeth and the driven apparatus from damage. Thus only the shear pin need be replaced in the coupling rather than the entire coupling or the driven device. Such couplings utilizing shear pins are known and are shown for example in Swanson U.S. Pat. No. 2,969,661 and Hochreuter U.S. Pat. No. 3,855,818. Both Swanson and Hochreuter disclose shear pins which are installed between adjacent coupling halves and when excessive torque is applied to one coupling half, the shear pin shears preventing torque from being transmitted from one coupling half to the other coupling half.
A problem associated with shear pin flexible gear type couplings is that the sleeve of the coupling half that surrounds the hub member is generally a floating type sleeve and the only contact between the sleeve and the hub is by the outer flange of the sleeve or in some designs between the intermeshing gear teeth of the coupling hub and sleeve. Conventionally, the intermeshing gear teeth between the sleeve and hub are located at the outer ends of each coupling half. This leaves a space between the sleeve and the hub which is axially spaced from the intermeshing gear teeth. This space is normally filled with a lubricant for lubricating the intermeshing gear teeth. Such a coupling is referred to as a floating sleeve flexible gear type coupling. The sleeve remains substantially concentric about the hubs when the sleeve of the individual coupling halves are secured together for the transmission of torque. When a shear pin is disposed between the sleeves of each half coupling, the floating sleeves remain concentric as long as no excessive torque shears the shear pin. When the shear pin shears, due to excessive torque, each sleeve is free to pivot about its outer flange or the intermeshing gear teeth and to axially move relative to the hub. When this happens, the sleeves are free to whip around radially and axially until the coupling halves come to rest causing a great amount of damage to the coupling in the interim between shearing and stopping. In addition, when the shear pin shears, both coupling halves continue to rotate for a certain period of time and since the sleeves are free to radially and axially whip around, contact can occur between each sheared portion of the coupling causing damage to the sleeves which may require replacement thereof.
Both Swanson and Hochreuter have disclosed devices for maintaining the sleeve substantially concentric around the hub until the coupling has come to rest after the shear pin has sheared. Swanson utilizes a shear pin adapter interposed between the opposed sleeves on the coupling halves. The adapter is constructed in two parts of which one part is secured to each sleeve and two parts of the adapter are secured together by a shear pin. When the shear pin shears, the adapter parts rotate about each other and prevent the sleeves from whipping around radially. In addition, Swanson provides a lock washer between the heads of the shear pins and the adapter so that when the shear pins shear they separate a small amount to prevent their contact while the coupling is still rotating. Hochreuter also provides a shear pin device between the sleeves of each coupling half. However, he provides a device wherein the sleeve of one coupling half rotates about the sleeve of the other coupling half in conventional ball bearings. When the shear pin shears between the coupling sleeves, one sleeve will rotate about the other sleeve without radial or axial movement between the sleeves. Both shear pin devices of Swanson and Hochreuter maintain the sleeve about the hubs substantially concentric when the shear pin shears and Swanson provides a means of separating the shear pin pieces so they do not contact each other after shearing. However, both devices are extremely expensive, require a high degree of machining, and require a large number of parts to be added to the coupling, the cost of which is naturally passed on to the customer.